1. Field of the Invention
The present invention relates to a procedure and apparatus for damping the vibrations of an elevator car or elevator car member supported by elastic suspension elements.
2. Description of Related Art
The degree of travelling comfort provided by an elevator is reduced by vibrations of the car. The vibrations induce unpleasant feelings in passengers and often result in undesirable noise.
Vibrations originating in the machine room of an elevator tend to be transmitted over the suspension ropes to the elevator car as vertical vibrations. The vibrations may be generated by sources like worn bearings of the machine, tooth wheels in the gear assembly or various auxiliary equipment such as tachometers.
In addition, electrical disturbances may generate oscillations in the elevator drive system. These oscillations may be transmitted as vertical vibrations via the motor, gear assembly, sheaves, pulleys and suspension rope to the elevator car. Moreover, horizontal vibrations caused e.g. by roughness of the guide rails, wear of the guide rollers or their bearings etc., may occur in the elevator car.
These undesirable vibrations of an elevator car typically have a frequency within the range from 1 Hz to 100 Hz and an amplitude of from 0.02 mm to 0.1 mm.
To damp undesirable vibrations, the rotating and sliding parts of the elevator machinery are manufactured to close tolerances, rotating parts are equilibrated and so on. Moreover, the elevator car may be insulated from the surrounding structures by using elastic suspension elements to support the car. All such arrangements are passive damping methods.
Active vibration damping systems are proposed in U.S. Pat. Nos. 4,030,580, 4,269,286 and 4,271,931. These employ a method whereby the elevator's speed reference is varied to damp the vibrations, the change in the speed reference being formed from the signal obtained from a tachometer installed in the elevator machine. However, the tachometer signal does not accurately represent the vibration of the elevator car, because e.g. vibrations caused by defects in diverter pulleys are not necessarily reflected in the tachometer signal.
Furthermore, a system based on varying the speed reference is best suited for damping car vibrations of a low frequency only, i.e. frequencies below 10 Hz, and is only applicable to the damping of vertical vibrations of the elevator car.